Pneumatic and electric fluid dispensers have been developed for dispensing applications requiring a precise placement of a fluid. Pneumatic dispensers have a significant advantage in that the pneumatic solenoid operating the dispensing valve provides a sufficient force so that the dispensing valve operation is essentially independent of the viscosity of the fluid being dispensed. However, pneumatic solenoids have disadvantages in that they generally have a shorter life than electric solenoids, and the operation of the pneumatic solenoid is subject to less precise control than the electric solenoid in an electric fluid dispenser. Therefore, in some applications, electrically operated fluid dispensers are preferred over pneumatic fluid dispensers.
Generally, electrically operated fluid dispensers include an electromagnetic coil surrounding an armature that is energized to produce an electromagnetic field with respect to a magnetic pole. The electromagnetic field is selectively controlled to open and close a dispensing valve by moving a valve stem connected to the armature. More specifically, the forces of magnetic attraction between the armature and the magnetic pole move the armature and valve stem toward the pole, thereby opening the dispensing valve. At the end of a dispensing cycle, the electromagnet is de-energized, and a return spring returns the armature and valve stem to their original positions, thereby closing the dispensing valve.
In the operation of an electric fluid dispensing gun, the coupling between the coil and the armature is not efficient; and therefore, in order to achieve the highest actuation speed, a current pulse or spike is typically provided to the coil during an initial turn on period in order to initiate the motion of the armature as quickly as possible. After the initial current pulse, the current through the coil is then reduced to approximately the minimum value required to hold the armature in its open position against the opposing force of the return spring. Such a stepped current waveform provides high performance while minimizing power dissipation in the coil.
The continued development and use of fluid electric dispensers has resulted in more demanding performance specifications. For example, the operational speed of the dispensing valve can be increased by increasing the electrical voltage applied to the electric coil operating the valve. However, simply doubling the applied voltage without other changes to the solenoid driver circuit would cause overheating and possibly degrade the performance of the fluid dispenser. For example, if a high voltage power supply is used with a low voltage solenoid driver circuit, the solenoid will switch proportionally faster. However, the low voltage solenoid driver provides an initial current pulse having more power than is required by the solenoid which results in an inefficient operation of the coil. Therefore, if it is desired to use a higher voltage to operate the fluid dispenser at a higher rate, not only must a new power supply be used; but a different solenoid driver circuit must be used. The requirement of replacing the whole driver circuit to upgrade the performance of the fluid dispenser is labor intensive, time consuming and expensive.
While the above problem has been described with respect to an electrically operated fluid dispenser, a similar problem exists with respect to pneumatically operated fluid dispensers. Therefore, there is a need to provide a fluid dispenser having a driver circuit which is operable with different power supplies.